1. Field of the Invention
The invention relates generally to micromachined structures and, more particularly, to micromachined structures in which a first frame is coupled to a plate or to a second frame by diametrically opposed torsion bars that permit rotation of the plate or second frame with respect to the first frame about a longitudinal axis of the torsion bars.
2. Background Art
A fundamental micromachined structure having many diverse applications consists of a first frame that is coupled to a plate or to a second frame by diametrically opposed torsion bars extending between the first frame and the plate or second frame to permit rotation of the first frame and the plate with respect to the second frame about a longitudinal axis of the torsion bars. Practical uses for this micromachined structure include optical beam torsional scanners, gyroscopes, flow meters, and profilometer and/or atomic force microscope ("AFM") heads, etc.
An example of a practical application for this structure is optical beam torsional scanners that are used in digital imaging, printing, bar code reading, optical recording systems, surface inspection systems, and various other scientific and industrial systems. In general, optical beam torsional scanners deflect a beam of light, usually from a fixed light source, over an angle ranging from several degrees to tens of degrees. Such optical scanners sweep a beam of light back-and-forth at a frequency determined in part by a mechanical resonant frequency of a reflecting mirror included in the scanner. A typical micromachined optical beam torsional scanner of the prior art is described in U.S. Pat. No. 4,732,440 to J. Gadhok. The concept of micromachining torsional optical beam torsional scanners within a silicon body was proposed at an early date by K. Peterson, Proc. IEEE, vol. 70, no. 5, p. 61, May 1982 ("the Peterson article"). See also U.S. Pat. No. 4,317,611 to K. Peterson.
FIG. 1, depicting an optical beam torsional scanner 30 shown in FIG. 39 of the Peterson article, includes a micromachined, inner torsional mirror plate 32, coupled to and supported by diametrically opposed, axially aligned torsion bars 34 within a outer frame 36. The aforementioned article describes typical scanner parameters, such as the modulus of silicon, the typical wafer thickness, the length of the torsion bars 34 and the dimensions of the mirror plate 32. The Peterson article describes the width of the torsion bars 34 as approximately 500 micrometers, while the length of the torsion bars 34 is approximately 0.2 centimeters. The mirror plate 32 is approximately 0.22 centimeters along each edge. A gap 38 which isolates the mirror plate 32 from the frame 36, and which also defines the torsion bars 34, is approximately 0.02 centimeters wide. The Peterson article describes that the gap 38 is fabricated by anisotropically etching a silicon wafer substrate.
The Peterson article further discloses that the frame 36 rests on a glass substrate 42 into which a cavity 44 has been etched, and that electrodes 46 have been vapor deposited onto the substrate 42. A linear support ridge 48 projects upward out of the cavity 44 to support the mirror plate 32 between the torsion bars 34. A high electrical voltage from a drive circuit (not depicted in FIGS. 1 and 2) applied first to one electrode 46 then the other in a continuing out-of-phase sequence causes the mirror plate 32 to rotate back-and-forth around a longitudinal axis 52. The electric fields generated by the electrodes 46 tilts the mirror plate 32 first to one side and then the other. A restoring force from the torsion bars opposes such rotation of the mirror plate 32. Although air damping affects slightly the resonance frequency approximately calculated for the mirror plate 32, a mechanical resonant frequency for rotation of the mirror plate 32 about the longitudinal axis 52 of the torsion bar 34 can be calculated with well known formulas cited in the above-mentioned articles. The torsional scanner 30 disclosed in the Peterson article includes the substrate 42, electrode 46 and drive circuit.
Gyroscopes that sense a rate of rotation by sensing the effects of a Coriolis force on an oscillating body are widely used for various applications. Though lacking the precision of rotary gyros, the simplicity and relative cost of rate gyros makes them attractive. One example of an application for rate gyros is the automotive brake control system in which the rate of rotation of the car needs to be sensed and controlled to avoid a spin.
Present prices for rate gyros are excessively high for many potential mass market applications such as automotive brake systems, robotic control, patient monitoring, virtual reality simulation, video games, video camera image stabilization, etc. One approach for achieving such a price reduction is to apply semiconductor fabrication techniques for fabricating rate gyros.
Micromachined rate gyro sensors have been made previously. U.S. Pat. No. 4,598,585, by B. Boxenhorn, assigned to Draper Laboratory ("the Boxenhorn patent"), describes a micromachined planar inertial sensor, consisting of a pair of gimbals, positioned at right angles to each other. The inner gimbal plate carries a substantial mass, which acts as the gyroscopic resonator. A gimbal frame surrounds the mass of the inner gimbal plate noted as the y-axis in the patent to support the inner gimbal plate via interconnecting torsion bars. The gimbal frame surrounding the mass of the inner gimbal is driven by electrostatic forces, and oscillates in a torsion mode about a longitudinal axis of the torsion bars at a frequency equal to the mechanical resonance frequency of the inner gimbal. Rotation of the rate gyro around the z-axis excites the surrounding gimbal's oscillation at its resonance frequency, which a set of capacitive sensors on the surrounding gimbal detect. The method disclosed in the Boxenhorn patent is elegant in principle.
In other prior art devices it is suggested that the gimbals may be made out of many materials, such as silicon dioxide, nitride, oxy-nitrides, or even stamped steel or aluminum sheets. However, it is very difficult to produce such materials with the proper stress by deposition. As a consequence, in these prior art gyros the mechanical resonant frequency of the inner gimbal is not well controlled during fabrication, and must, therefore, be trimmed after fabrication to match the driving frequency of the surrounding gimbal. The materials proposed for such micromachined rate gyros are also subject to work hardening. Consequently, the resonant frequency of the inner gimbal changes over time, causing a mismatch with the resonant frequency of the surrounding gimbal, and an apparent loss of rate gyro sensitivity.
U.S. Pat. No. 4,699,006 by B. Boxenhorn discloses an oscillating digital integrating accelerometer, based on the same technology as that described above for the rate gyro. In the digital accelerometer application, a z-axis acceleration causes a change in the resonant frequency around the gyro's y-axis. The changes in frequency indicate z-axis acceleration.
U.S. Pat. No. 5,016,072 by Paul Greiff ("the Greiff patent") describes further improvements on the rate gyro structure disclosed in the Boxenhorn patent. The Greiff patent replaces dielectric layers disclosed in the Boxenhorn patent with a sheet of boron doped p+ silicon, and replaces the asymmetric mass of the Boxenhorn patent with a symmetric one. However, in the rate gyro disclosed in the Greiff patent buckling of the inner torsion bars causes undesirable large variations in the resonant frequency of the inner gimbal, and requires special torsion bar footings. The Greiff patent also requires grooves for the torsion bars to provide controllable stiffness. The boron doped silicon material requires stress relief and trimming of the torsion bars. Moreover, electrostatic force balancing techniques are needed to restrain motion of the inner gimbal, and to avoid cross-coupling between the inner and surrounding gimbal and changes of resonant frequency.
U.S. Pat. No. 5,203,208 by J. Bernstein ("the Bernstein patent") also assigned to Draper Lab, describes a symmetric micromechanical rate gyro, also using boron doped silicon. Here the mechanical resonance frequencies of both gimbals are designed to be the same, and trimmed to be identical. Trimming slots are also required in the gimbals to relieve stress in the boron doped silicon. Consequently, the rate gyro disclosed in the Bernstein patent requires vastly reduced drive voltage which substantially reduces parasitic pick-up signals.
Boxenhorn and Greiff in Sensors and Actuators, A21-A23 (1990) 273-277 (the Boxenhorn et al. article) describe an implementation of a silicon micromachined accelerometer of the type described in U.S. Pat. No. 4,598,585, mentioned above. The Boxenhorn et al. article again describes fabricating torsion bars from boron diffused silicon. The Boxenhorn et al. article ascribes difficulties encountered with this approach to unknown pre-stress, which sets the stiffness of the torsion bars and sensitivity of the device.
One of the problems encountered in prior art structures of the type described above is restricting oscillations of a frame or mirror plate 32 to a single desired torsional mode of rotation about the longitudinal axis 52. In all the processes for fabricating rate gyros described in the references identified above, stresses in the torsion bar material are uncontrolled, and the mechanical resonant frequencies unpredictable. Another deficiency in the prior art structures is sensing relative rotation about the longitudinal axis of diametrically opposed torsion bars that couple one frame or plate to a frame. Yet another difficulty with the prior art structures and fabrication methods is an inability to control, balance, or eliminate stress in micromachined plates or frames.